The invention relates to a process for the treatment of liquid septage and biosolids. In particular, this invention describes a process whereby alkaline material is mixed with liquid septage, the septage is de-watered, the de-watered solids are pasteurized, and the end product meets the Class A vector attractant reduction and pathogen reduction requirements mandated by the U.S. EPA. A process for treating liquid septage and biosolids is also disclosed whereby alkaline material is mixed with liquid septage, the septage is de-watered, and the end product meets the Class B pathogen reduction requirements mandated by the U.S. EPA.
With the increasing number of homes that utilize septic tanks, the need for treating liquid septage has increased substantially in recent years. Untreated septage, both in solid and liquid form, may contain any number of noxious substances that are harmful to humans and the environment. These include particulate solids, organic and inorganic compounds, and pathogens.
There is developing an increasing demand that the treatment of septage be sufficiently thorough to allow beneficial re-use of septage, such as enabling the septage to be applied to land on which agricultural and ornamental crops may be grown. While septage has beneficial plant nutrients, it also may contain bacteria, viruses, protozoa, parasites, and other microorganisms which may be disease causing. In an effort to make stabilized septage more marketable, those treating the septage have attempted to produce a granular, scatterable product having a soil-like texture.
Because septage is identical to municipal sewage sludge, processes and methods used to treat sludge can also be used to treat septage, and nearly all EPA regulations that apply to sewage sludge also apply to septage. One way of treating sludge is to mix alkaline products with the sludge in a manner that blends the alkaline and sludge and transforms the otherwise watery sludge into an acceptable end product. With the development of modern sewage systems, the use of lime products for flocculation of solids in raw sewage or liquid sludge has been developed. The role of alkaline materials in pH adjustment and the beneficial effect of pH in pathogen reduction is a more recent development, having occurred in only the last 60 years.
The end result of research into pathogen reduction in wastewater treatment has prompted the U.S. Environmental Protection Agency to promulgate regulations specifying environmentally sound treatment criteria. More specifically, the EPA""s standards for the use or disposal of sewage sludge (40 CFR 503; xe2x80x9cMethods for Treatment and Disposal of Sewage Sludge,xe2x80x9d) establishes methods for achieving pathogen and vector attraction reduction using alkalinity (pH) and temperature criteria.
The aforementioned EPA regulation outline a number of chemical processes allowed to treat sludge to render it suitable for beneficial use in agriculture and similar application. The regulations establish two classes of sludge treatment products: Class A and Class B. A Class A product can be achieved by heating the sludge to no less than 50xc2x0 C. for no less than 30 minutes while simultaneously raising the product pH to no less than 12 and keeping this pH level for at least 2 hours, followed by a pH of 11.5 for 22 hours. Adding quicklime to liquid or de-watered sludge can achieve the aforementioned pH, temperature and time conditions.
A majority of research in this field involves the pathogenic stabilization of sludge through pH adjustment and heat generated from the exothermic reaction of quicklime with water. U.S. Pat. No. 4,270,279 issued to Roediger and U.S. Pat. No. 4,306,978 issued to Wurtz disclose methods utilizing such research. U.S. Pat. No. 4,270,279 discloses the gentle handling of partially de-watered sludge cake and dusting only the surfaces of sludge particles thus resulting in a granular product; however, this process can only be carried out using partially de-watered sludge cake in sheet form prepared by belt filter presses.
Lime is the major expense in the lime treatment process. In order to meet current Class A requirements mandated by the U.S. EPA, some sludge treatment processes require the use of significant amounts of lime. It is not uncommon to use between 900 pounds and two tons of lime per ton of dry solids, depending upon moisture content of the incoming sludge (usually between 15% and 35% total solids) and the end product dryness required for beneficial use (usually 45% to 60% total solids).
One further method to treat waste to kill pathogens is to heat it to a high temperature for a period of time. Commonly known as pasteurization, this process neutralizes pathogens to a degree dependent upon the level of temperature and length of time that the waste is exposed to the elevated level. Where both pasteurization and the addition of an alkaline added in combination are performed, pasteurization temperatures can kill pathogenic organisms while the alkaline additive can prevent regrowth of organisms. The result can be an end product that can be stored for extended periods of time. If satisfactorily processed in this manner, sludge may be applied to lands without need for site-specific permits, and thus may be marketed, distributed, and sold as fertilizer.
To properly ensure complete pathogen reduction, and to meet the Class A requirements mandated by U.S. EPA, the heated sludges must be held at no less than 50xc2x0 C. for no less than 30 minutes. Since these sludges are exempt from many restrictions for land application, EPA has stated that the time-temperature requirements apply to every particle of sludge processed.
Currently, there are several known methods wherein alkaline material is added to the sludge and the sludge is subjected to additional heat. U.S. Pat. Nos. 5,275,733 and 5,417,861, both issued to Burnham, disclose a method for treating de-watered sludge. This method discloses adding alkaline material to de-watered sludge and then subjecting the sludge to additional heat for drying and sterilization. U.S. Pat. Nos. 5,681,481; 5,783,073; and 5,851,404; all issued to Christy et al., disclose a method for treating sludge wherein alkaline material is added to a substantially liquid sludge and the sludge is then subjected to additional heat.
Both of these methods are inefficient in that adjusting the pH level of de-watered sludge requires more alkaline material than needed to adjust the pH level of liquid sludge. Adjusting the pH level of de-watered sludge also requires more energy to mix in the alkaline material than needed when treating liquid sludge. Conversely, adding heat to pasteurize liquid sludge requires more energy than pasteurizing sludge after it is de-watered.
Currently, there are several devices commercially available that attempt to address the inefficiency of pasteurizing liquid sludge by combining de-watering and pasteurization during the treatment process. However, these devices still must heat at least some of the liquid in the sludge. Additionally, the production rate of this process is slower than other processes due to the de-watering step being slowed down so the sludge can meet the EPA mandated time and temperature requirements for pasteurization.
Accordingly, there is clearly a need for a process that can meet the EPA Class A pH, and pasteurization requirements while minimizing the amount of alkaline material and energy needed to carry out the process. Moreover, there is a need for a process which efficiently and effectively achieves a virtually pathogen free end product which is unsuitable for the regrowth of undesirable organisms.
While the term septage is commonly used to describe the material removed from septic tanks, the invention disclosed herein can also be used to treat other biosolids including but not limited to, waste removed from portable toilets, sewage sludge, and other types of sludge. As used herein, the term septage should be read to include other types of biosolids, and the term biosolid should be read to include septage.
It is a primary object of the present invention to provide a process for treating liquid septage and other organic wastes (biosolids).
It is also an object of this invention to provide a process that overcomes the inefficiencies associated with the prior art.
Another object of the present invention is to provide a novel process that uses less alkaline material, than the prior art processes, to meet the Class A vector attractant reduction and pathogen reduction requirements mandated by the U.S. EPA.
It is a further object of the present invention to provide a novel process that uses less energy for pasteurization, than prior art processes use to meet Class A vector attractant reduction and pathogen reduction requirements mandated by the U.S. EPA.
Yet another object of this invention is to provide such a process that is cost effective and easy to implement.
These, as well as additional objects of the present invention, are attained by mixing the liquid septage with sufficient alkaline material to raise the pH level of the septage to 12 or greater for a minimum of 2 hours and hold it at 11.5 pH or greater for an additional 22 hours.
While the preferred alkaline material for this process is quicklime, other materials including hydrated lime, limestone, fly ash, wood ash, sodium or potassium hydroxide, kiln dust, etc., can also be used. The mixing of alkaline material with the liquid septage is achieved by metering lime into the septage and using the turbulence generated in the re-circulating pump, plumbing and holding tank to mix the lime and septage. Mixing can also be accomplished by agitating the liquid with air or mechanical devises.
The pH-adjusted septage is then stored for a period of at least 24 hours and the pH level is monitored to ensure that it meets the Class A vector attractant reduction and pathogen reduction requirements mandated by the U.S. EPA.
The pH-adjusted septage can be mixed with a polymeric flocculent either before or after the storage process. The mixing of the flocculent and the pH-adjusted septage can be achieved by agitating with mechanical devises, or turbulence of the liquid. The flocculent thickens the septage by binding to the solids and causing the solids to settle to the bottom of a storage tank such that the liquid effluent can be decanted off of the top. The liquid effluent is then pasteurized by heating it to a temperature and held at that temperature for sufficient time to meet the Class A vector attractant reduction and pathogen reduction requirements mandated by the U.S. EPA.
The flocculated pH adjusted septage blend is fed into a de-watering device where the free liquid is separated from the solids and drained away. The septage is further subjected to pressure to remove additional liquid producing a de-watered solids cake that ranges from 15% to 50% solids or 85% to 50% water content. While the preferred de-watering device is a screw press, other de-watering devises including belt presses, centrifuges, or rotary presses, etc. can be used. The liquids separated from the septage during de-watering can be mixed with the liquid effluent removed after flocculation if it does not contain a very high percentage of solids. If the liquids removed during de-watering contain a high percentage of solids, it is added to a subsequent batch of septage and repeats the process.
The de-watered solids cake is discharged from the screw press and conveyed into pasteurization equipment where it is heated to a temperature and held at that temperature for sufficient time to meet the Class A vector attractant reduction and pathogen reduction requirements mandated by the U.S. EPA.
In other embodiments of the current invention, the septage is de-watered before it is stored, and flocculation occurs immediately before it is de-watered. The de-watered solids cake and the liquid effluent can then either be pasteurized and then placed in a storage area for monitoring of the pH level, or stored and then pasteurized at the completion of storage.
The advantage of the above process is lower alkaline material dosage to achieve the required pH, higher production rates from the screw press, and lower energy costs for pasteurization. The average amount of quick lime used per dry ton of solids in this process is 300 pounds, while that used in processes that require mechanical mixing or where the quick lime is applied to de-watered septage is 400-1000 pounds per-dry ton. When commercially available screw presses are not used for pasteurization, it is possible to increase the production of septage de-watered cake by up to 100% without an offsetting increase of cake moisture content. Additionally, the use of an enclosed pasteurization vessel for the de-watered solids and a heat exchanging pasteurization device for the liquid effluent, is more efficient than pasteurizing the solids and liquids together, and therefore saves energy costs.